Proprietary trading and investments in hedge funds and private equity funds, like other trading and investment activities, provide banking entities with revenue but also create the potential for losses. Banking entities have conducted proprietary trading at stand-alone proprietary-trading desks but also have conducted such trading elsewhere within their firms. GAO determined that collecting information on activities other than at stand-alone proprietary trading desks was not feasible because the firms did not separately maintain records on such activities. As a result, GAO did not analyze data on broader proprietary trading activity but analyzed data on stand-alone proprietary-trading desks at the six largest U.S. bank holding companies from June 2006 through December 2010. Compared to these firms' overall revenues, their stand-alone proprietary trading generally produced small revenues in most quarters and some larger losses during the financial crisis. In 13 quarters during this period, stand-alone proprietary trading produced revenues of $15.6 billion--3.1 percent or less of the firms' combined quarterly revenues from all activities. But in five quarters during the financial crisis, these firms lost a combined $15.8 billion from stand-alone proprietary trading--resulting in an overall loss from such activities over the 4.5 year period of about $221 million. However, one of the six firms was responsible for both the largest quarterly revenue at any single firm of $1.2 billion and two of the largest single-firm quarterly losses of $8.7 billion and $1.9 billion. These firms' hedge and private equity fund investments also experienced small revenues in most quarters but somewhat larger losses during the crisis compared to total firm revenues. Losses from these firms' other activities, which include lending activities and other activities that could potentially be defined as proprietary trading, affected their overall net incomes more during this period than stand-alone proprietary trading and fund investments. Some market participants and observers were concerned that the act's restrictions could negatively affect U.S. financial institutions by reducing their income diversification and ability to compete with foreign institutions and reducing liquidity in asset markets. However, with little evidence existing on these effects, the likelihood of these potential outcomes was unclear, and others argued that removing the risks of these activities benefits banking entities and the U.S. financial system. Financial regulators have struggled in the past to effectively oversee bank holding companies. While the act's restrictions reduce the scope of activities regulators must monitor, implementing them poses challenges, including how to best ensure that firms do not take prohibited proprietary positions while conducting their permitted customer-trading activities. Regulators have yet to gather comprehensive information on the extent, revenues, and risk levels associated with activities that will potentially be covered, which would help them assess whether expected changes in firms' revenues and risk levels have occurred. Without such data, regulators will not know the full scope of such activities outside of stand-alone proprietary trading desks and may be less able to ensure that the firms have taken sufficient steps to curtail restricted activity. As part of implementing the new restrictions, regulators should collect and review more comprehensive information on the nature and volume of activities potentially covered by the act. Treasury and the financial regulators agreed to consider this as part of their rulemaking.